The present invention relates in general to a flame sensing circuit. More particularly, the present invention relates to an apparatus useful for detecting a flame in the combustion chamber of a water heater or boiler by sensing the infrared components of the light emanating from the flame.
Many methods of detecting a flame are known. U.S. Pat. No. 2,911,540 to Powers discloses a flame detection system incorporating a photoconductive cell such as a cadmium sulfide cell which is electrically sensitive to flame. The cell is connected through a high resistance to a direct current voltage source in such a manner that when and as the intensity of the radiation increases, the voltage across the cell decreases. The voltage which is developed across the cell may be considered to consist of two components. The first component is a substantially continuous unidirectional voltage representative of the average light intensity of the radiation and the second component varies in accordance with sporadic fluctuations in the intensity of such flame. The frequency of flame fluctuation is approximately 25 Hz. The voltage developed across the cell is applied to a unique amplifying system which is tuned to a frequency of approximately 25 Hz so as to selectively amplify that component of the voltage representative of the sporadic fluctuations.
U.S. Pat. No. 3,727,073 to Cade discloses a flame sensor control circuit. The preferred embodiment of this invention features a control apparatus for a burner. Cade utilizes a flame rod detector that provides a signal to an electronic amplifier, which in turn operates multiple relays responsive to the flame rod detector signal.
U.S. Pat. No. 3,742,474 to Muller discloses a flame detector. The Muller flame detector comprises a photosensitive transducer such as a photocell, a photodiode or a photoresistance located so as to be sensitive to the radiation omitted by the flame. The phototransducer is designed to provide an electrical output signal. The remainder of the detector circuit is designed to indicate the presence of a flame by producing an alarm or control signal. Muller discloses a device that discriminates between radiation from a flame and from spurious radiation by selecting the higher proportion of infrared radiation which is present when flames arise due to a fire. A red sensitive and blue sensitive photocell are serially connected, and an electrical circuit arranged so that an alarm signal is derived only when the red-blue ratio exceeds a predetermined value. The Muller device includes a filter for passing frequencies within a predetermined bandwidth, for example 5 to 25 hertz or 2 to 40 hertz.
U.S. Pat. No. 3,820,097 to Larson discloses a flame detection system with compensation for the flame detector. Larson teaches the use of a flame responsive impedance, such as a lead sulfide photocell, which responds to a change in infrared radiation and the flame flicker frequency of a sensed flame. The flame flicker frequency is normally in the 6 to 15 Hertz range. The system normally is tuned by a bandpass amplifier in a range up to approximately 15 to 18 Hertz. The flame flicker signal is amplified and used to control an output switch.
U.S. Pat. No. 4,395,638 to Cade discloses a self-checking flame failure control of the type which continuously monitors its own performance so as to be self-checking, i.e., to produce a response whenever the system shows the absence of a condition being monitored or a change therein. Cade also produces a response if the control system itself should become inoperative or otherwise malfunction. Cade teaches the use of two sensors capable of sensing the same event and producing two separate but comparable signals indicating the presence of the event and, conversely, the absence of such signal to indicate the absence of the event. Two photodetectors are respectively connected in series with resistors which are connected in series across an appropriate DC power supply. The photo detectors effectively change resistance upon reception of radiation energy. Comparable signals appear on two separate lines when the same event, i.e., the presence of a flame, is sensed by the photodetectors. The comparable signals are compared by a voltage comparator, the output of which is amplified and applied to a Schmidt trigger. If a flame is present, the comparison will indicate comparable signals.
U.S. Pat. No. 4,591,725 to Bryant discloses a system for amplifying all frequencies present in a signal detected from a flame detector. A photodetector provides a signal corresponding to flame intensity and includes signal components attributable to flame flicker. A photovoltaic light detector, usually a silicon diode, is employed to generate a signal in response to light impinging upon it. When the light comes directly from the axial midportion of a flame, the intensity of the light will vary according to a flicker frequency and, therefore, the signal from the detector has the flicker frequency superimposed on it. All frequencies down to D.C., including flame flicker frequency as well as D.C., are amplified equally. The signals are then processed further downstream in order to isolate the flicker frequencies for the purpose of indicating a flame-on condition in the conventional manner.
U.S. Pat. No. 4,639,717 to DeMeirsman discloses a method and apparatus for monitoring flame conditions. This reference discloses a flame monitor that senses flame brightness and produces two signals corresponding to rapid brightness variations and average brightness. By comparing rapid signal variations to a fraction of the average signal, a threshold ON-OFF signal representing normal flame operation is obtained that can be processed as a fail-safe indication and control. High and low limit thresholds can be set and compared with average brightness as a further condition of proper flame operation.
U.S. Pat. No. 4,904,986 to Pinckaers discloses an infrared flame amplifier. A flame sensor circuit controls the flow of fuel to the burner or boiler. A photocell is used to produce a flame signal when the photocell is exposed to a flame. An input circuit is coupled to the photocell for receiving and buffering the flame signal. The buffered flame signal is filtered and amplified in a filter. An output circuit further amplifies the filtered flame signal providing an output flame signal. Pinckaers teaches the use of an infrared sensitive lead sulfide cell for detecting a flame and producing a corresponding flame signal. Switch means are coupled to the lead sulfide cell for substantially short circuiting the flame signal to a known value upon receiving a switch-close signal. Hot refractory detection means are also coupled to the photocell for detecting a flame-out condition and generating a hot refractory detection signal causing the flame signal to be substantially short circuited to a known value thereby eliminating a false flame signal caused by hot refractory shimmering.
German Patent DL-140-170 to Rauschenbach discloses a circuit used to detect the presence of a flame. The Rauschenbach device includes an optoelectronic component, namely a phototransistor, a MOSFET amplifier and an RC element filter with a time constant tuned to the flame ignition frequencies. The sensitivity of the filter is greatest at the flame frequency range to minimize the effects of stray radiation pickup.
A more economical and easily manufactured flame sensor is needed for applications wherein remote flame detection is desired.